Brake control system utilizing fluidic logic elements

ABSTRACT

A pure fluid type brake control system for railway vehicles in which an input control signal representative of the desired brake effort is modified in accordance with the vehicle load for controlling the dynamic brakes. The difference between the load weighed brake control signal and a feedback signal corresponding to the variable effectiveness of the dynamic brake establishes an error signal for control of the friction brake which is regulated through a separate feedback loop to continuously blend with the dynamic brake and thereby satisfy the load weighed brake control signal. Provision is made to obtain full friction brake in response to a malfunction occurring in the digital application and release network comprising the friction brake control independently thereof.

[451 Apr. 4, 1972 BRAKE CONTROL SYSTEM UTILIZING FLUIC LOGIC ELEMENTSRonald A. Sarbach, Columbus, Ohio Westinghouse Air Brake Company,Wilmerding, Pa.

[22] Filed: Apr. 28, 1970 [21] Appl. No.: 32,646

[72] Inventor:

[73] Assignee:

[52] US. Cl. ..303/22 R, 188/195, 303/20 [51] lnt. Cl. ..B60t 8/22 [58]Field of Search ..188/156, 195; 303/3, 20, 21, 303/22 R, 22 A [56]References Cited UNITED STATES PATENTS 3,398,815 8/1968 Brath et a1...303/22 X 3,398,993 8/1968 Sarbach et a1. ..303/22 X 3,443,842 5/1969Pier ..303/22 A X 3,490,814 1/1970 Smith et a1 ..303/22 X TRANSDUCERBlAS TRANSDUCER 3,547,499 12/1970 Maskery ..303/22X PrimaryExaminer-Milton Buchler Assistant Examiner-Stephen G. KuninAttorneyRalph W. Mclntire, Jr.

[57] ABSTRACT A pure fluid type brake control system for railwayvehicles in which an input control signal representative of the desiredbrake effort is modified in accordance with the vehicle load forcontrolling the dynamic brakes. The difference between the load weighedbrake control signal and a feedback signal corresponding to the variableeffectiveness of the dynamic brake establishes an error signal forcontrol of the friction brake which is regulated through a separatefeedback loop to continuously blend with the dynamic brake and therebysatisfy the load weighed brake control signal. Provision is made toobtain full friction brake in response to a malfunction occurring in thedigital application and release network comprising the friction brakecontrol independently thereof.

8 Claims, 1 Drawing Figure FROMMR.

BRAKE CONTROL SYSTEM UTILIZING FLUIDIC LOGIC ELEMENTS BACKGROUND OF THEINVENTION Improved high speed rapid transit railway systems are becomingincreasingly important in todays highly mobilized society fortransporting large masses of the populace between inter-urban points andalong densely populated corridors between cities. The ability to achievehigh speed transit service is dependent to a large extent upon thevehicle braking system, especially where the adequacy of the servicerequires that minimum train headways and tight operating schedules aremaintained without discomfort to passengers. Electronic brake controlsystems, which offer fast response and continuous automatic regulationof the retardation effort, together with other brake systemdevelopments, have been responsible in upgrading the brake systemtechnology to meet existing high speed mass transit requirements.

Being fragile by nature, sensitive to environmental conditions such aselectrical interference and extreme temperature variations, complex bynecessity of design and requiring a sophisticated power supply, it isapparent, however, that these electronic brake control systems areexpensive both in initial cost and to maintain, especially under suchadverse conditions as are known to exist in railroad operations as weknow them today.

Conversely, the recently evolved pure fluid technology known as fluidicsemploys fast responding components which have no moving parts to wearand therefor exhibit a long, service free operating life, which arereliable under extreme temperature variations, and are substantiallyinsensitive to stray electrical fields, in addition to utilizing thepneumatic medium as a source of power which is readily available in theair supply system on rapid transit type railway cars.

SUMMARY OF THE INVENTION It is therefore the principle object of thepresent invention to provide a brake control system having sensitive andnear instantaneous response for automatically regulating the brakeeffort of a railway vehicle including the functions of variation ofbraking level with vehicle load and continuous dynamic/friction brakeblending.

It is another object of the invention to provide a vehicle brake controlsystem having a failsafe arrangement in which a redundant circuit checkis provided to monitor the release elements of the friction brakecontrol network so that in the event a malfunction arises therein,application of the friction brake will occur independently of the normalfriction brake control.

It is yet another object of the invention to provide a vehicle brakecontrol system, as outlined above, comprised of pure fluid controlelements.

In the present invention, these objects are achieved by feeding ananalog fluid pressure control signal in the fluidic range of pressurelevels from an appropriate transducer to a proportional type pure fluidamplifier. The transducer is selected in accordance with the medium ofthe control system input signal employed and converts this analog inputsignal to a proportional fluidic signal representing the desiredretardation effort. The output of the proportional amplifier reflectsthis desired retardation signal which is subsequently modified inresponse to variation of vehicle load through automatic adjustment of anorifice connecting the amplifier output to atmosphere. Dynamic brakecontrol responsive to the load weighed control signal is compared by asecond pure fluid proportional amplifier with a feedback signalcorresponding to the effectiveness of the dynamic brake which istypically slow in responding and which fades as the vehicle slows. Theresulting difference between signal levels establishes an error signalat the amplifier output representing the amount of friction brakenecessary to satisfy the degree of brake retardation requested.

Each truck of a rapid transit vehicle is provided with digitalapplication and release control circuits subject to the error signal forcontrolling the friction brakes. A pair of pure fluid Schmitt Triggercontrol circuits in the respective application and release networksrespond to variation in the error signal to produce output signals forcontrolling two-way, low to high pressure pneumatic relay valves so thatwhen the Schmitt Trigger in the application circuit responds to theerror signal, a supply of high fluid pressure is connected via one relayvalve to the brake units and when the Schmitt Trigger in the releasecircuit responds, the brake application pressure is exhausted via theother relay valve. A signal corresponding to the fluid pressureeffective at the brake units acts on a third pure fluid proportionalamplifier, the output of which is connected as a feedback signal to theapplication and release Schmitt Triggers to provide system regulation.Opposing the error signal at the Schmitt Triggers, the feedback signalwill disable the appropriate application or release circuit whichinitiated the supply or exhaust of brake pressure, as the case may be,and consequently produce a lap condition of the friction brake.

A redundant circuit arrangement including a NAND gate common andresponsive to corresponding signals from the pure fluid release circuitsassociated with each truck of the vehicle monitors the relativecondition of the respective truck release circuits, causing applicationof the friction brake if the control signals fail to agree after apredetermined delay due to one of the release circuits malfunctioning.An OR gate interposed in the application network of each truck betweenthe relay valve and Schmitt Trigger to which one of its control ports isconnected is subject at its other control port to the output of the NANDgate to override the normal brake control of the application SchmittTrigger when a malfunction occurs.

Other objects, features and attendant advantages of the presentinvention will be apparent from the following more detailed descriptionand operation when considered with the accompanying diagrammatic drawingillustrating the invention.

DESCRIPTION AND OPERATION Referring now to the drawing, the systembasically includes an input transducer 1; pure fluid proportional typeamplifiers 2 and 3 for establishing a load weighed, dynamic brakemodified, friction brake control signal; redundant digital frictionbrake control circuitry 4 and 5 associated with the railway truckssupporting opposite ends of a railway vehicle; fluid pressure operatedbrake means such as typical friction brake units indicated by blocks 6and 7 responsive to the control circuits 4 and 5 and normally mounted onthe vehicle truck for applying a friction brake force in the area of thewheels thereof; and an AND/NAND gate 8 common to circuits 4 and 5 forproviding failsafe operation of circuits 4 and 5.

Transducer 1 is selected in accordance with the nature of the systeminput signal effective at a control port 9 to convert the input signalinto a proportional pneumatic fluidic pressure signal.

Proportional amplifiers 2 and 3 each include a supply port to which asource of fluidic pressure is connected, a pair of opposing controlports 10 and 11, and a connected output 12. Control port 10 of amplifier2 is connected to output 13 of transducer 1 while opposing control port11 is supplied with a bias signal which is adjusted and maintained at alevel sufficient to drive the amplifier power stream away from output 12when the signal at control input 10 is absent so that no output signalis provided. Output 12 of amplifier 2 is connected to atmosphere by wayof a pair of series connected fluid pressure flow restrictors 14 and 15which form a pressure divider. The orifice of restrictor 15 is variedwith vehicled load by means (not shown) to control the pressure levelbetween the restrictors depending upon the magnitude of the signal atoutput 12. Thus, as the brake control signal at the input transducer 1is varied, its output signal, which is converted by the transducer tothe proper pneumatic medium, is utilized to control amplifier 2, theoutput of which is varied accordingly and thence modified to reflect thechanging load conditions of the vehicle in consequence of adjustment ofthe variable orifice of restrictor 15.

Connected to the input of a pneumatic (fluidic) to electric transducer16 having an output adapted to regulate the dynamic brake controlcircuitry (not shown) and to input port of amplifier 3 is the loadadjusted fluid pressure brake control signal effective between seriesrestrictors l4 and 15. The opposite control input 11 of amplifier 3 isconnected with the output of an electric to pneumatic (fluidic)transducer 17 responsive to the dynamic brake control circuits formonitoring the effectiveness of the dynamic brake. The pressuredifferential across control ports 10 and 11 of amplifier 3 therefore isrepresentative of the difference between the brake request and thedegree of dynamic brake being provided. In accordance with conventionalpure fluid type proportional amplifier operation, a proportional fluidpressure error signal representative of this difference is produced atoutput 12 of amplifier 3 for establishing the degree of friction brakecontrol necessary to satisfy the brake request signal at input 9 oftransducer 1. It will be apparent that as the dynamic brake feedbacksignal effective at input 11 of amplifier 3 becomes progressively lessin accordance with the fade characteristic of the dynamic brake as thevehicle approaches a stop, the error signal at output 12 becomesproportionally greater to provide the degree of friction brake necessaryto maintain a level of brake effort consistent with the brake requestsignal through continuous blending of the separate dynamic and frictionbrake control systems. Conversely, as the error signal (hereinafterreferred to as the friction brake control signal) between the desiredand actual brake effort realized from the dynamic brake decreases, dueto the dynamic brake providing a greater share of the braking, thefriction brake demand required to satisfy the brake request becomesproportionally less. If the dynamic brake is able to provide the totalbrake request, there will be no pressure signal at output 12 ofamplifier 3 and the friction brakes will be withheld.

Friction brake control circuits 4 and 5 are identical in both structureand function for installation on the separate trucks supporting oppositeends of a railway vehicle and will therefore be described with referenceonly to circuit 4, it being understood that the description also appliesto circuit 5.

Brake control circuit 4 is comprised of application and release controlnetworks, which include a pure fluid Schmitt Trigger circuit 18 and 19as the input elements responsive to the friction brake control signalfor controlling the application and release control network componentsand low to high pressure digital relay valve device 20 and 21respectively. A pure fluid proportional amplifier 22 is utilized as thefeedback element for producing a signal at Schmitt Triggers l8 and 19representative of the friction brake force developed. Also provided inthe application network is a pure fluid OR gate 23 interposed betweenthe Schmitt Trigger 18 and relay valve 20. The low to high pressurerelay valve devices 20 and 21 may be one of several commerciallyavailable types suitable for the intended application of producing ahigh pressure output in response to a low or fluidic control signal. Forpurposes of simplifying the discussion, the low to high pressure relayvalve devices 20 and 21 may be considered as providing a two-way low tohigh pressure conversion function; i.e., the valve either establishes orinterrupts fluid pressure communication between its supply and deliveryports in accordance with the pressure or absence of a low pressure(fluidic) control signal.

The identical pure fluid Schmitt Trigger circuits l8 and 19 areillustrated diagramatically but in actual practice may be fabricatedfrom an input flip-flop device connected through three stages ofcascaded proportional amplifiers to an output OR/NOR gate. Therepresentative symbology illustrates this circuit which includes a pairof supply ports to which separate sources of fluidic supply pressure areconnected, a pair of opposing control ports 24 and 25, and outputs 26and 27. Being,

in reality, a highly sophisticated flip-flop or bistable device capableof being easily switched at prescribed pressure levels due to its highsensitivity, pressurization of control port 24 at a predeterminedminimum pressure level above the pressure level at opposing input 25will result in the Schmitt Trigger circuit switching on to pressurizeoutput 26, which will remain pressurized by inherent memory of thedevice until the control port 25 subsequently becomes pressurized atsome value above pressurization of port 24 to consequently switch offthe device, in which state output 26 becomes depressurized until thedifferential across the control ports is again'reversed The frictionbrake control signal from output 12 of amplifier 3 representing therequired friction brake is connected to control port 24 of SchmittTrigger 18 associated with the application network and also to controlport 24 of Schmitt Trigger 19 associated with the release network ofbrake control circuit 4. Assuming the absence of a control signal atopposing control ports 25 of the respective Schmitt Triggers 18 and 19,as will be the case when the brakes are in a fully released condition,control pressure at inputs 24 will be effective to cause pressurizationof outputs 26 of each Schmitt Trigger.

Output 26 of application Schmitt Trigger 18 is connected to a controlport 28 of OR gate 23 so that when pressurized, the power stream of ORgate 23 is deflected so as to pressurize its OR output 29. Outputs 26 ofrelease Schmitt Triggers 19 associated with brake control circuits 4 and5 are connected to control ports 30 and 31 respectively of AND/NAND gate8. Concurrent pressurization of control ports 30 and 31 causes AND/NANDgate 8 to depressurize its NAND output 32 connected to a control port 33of OR gate 23. This is the normal condition of the circuits with theabsence of a control signal at input 33 of OR gate 23 allowing fullcontrol of OR gate 23 by application Schmitt Triggers 18 for control ofthe brakes.

Pressurization of OR gate control port 28 by Schmitt Trigger output 26as just described results in its supply stream being switched into ORoutput 29 connected to the control port 34 of digital relay valve 20used as a supply valve. This results in valve 20 being enabled toestablish fluid pressure communication between a regulated high pressuresource of supply effective at input port 35 and the brake units 6connected to output port 36. As long as control port 34 remainspressurized to pilot supply valve 20, fluid pressure will build up atthe brake units and friction brake forces will develop accordingly, theforces developed being dependent therefore upon the duration SchmittTrigger l8 continues to enable OR gate 23. It will be noted that duringthis period of brake application, control port 34 of relay valve 21,which is identical to valve 20 and is used as an exhaust valve in therelease network, is depressurized due to its being connected to ventedoutput 27 of Schmitt Trigger 19 in the release network. Consequently,its input port 35, which is connected to the brake units 6 is cut offfrom output port 36 which is connected to atmosphere. Thus, pressuredelivered to the brake units 6 by supply valve 20 is prevented fromexhausting to atmosphere during periods of brake application.

Subject to the buildup of fluid pressure at the brake units via a choke37, which reduces the high pressure to a proportional analog signal atfluidic levels, is control port 10 of proportional amplifier 22 which isidentical with previously described amplifiers 2 and 3 and is identifiedwith corresponding reference numerals. A bias signal is provided atcontrol port 11 in opposition to the signal effective at port 10sufficient to maintain substantially zero pressure at output 12 whenbrake application pressure is reduced below a value of any consequence.As brake application pressure develops, the corresponding signalincrease at port 10 overcomes the bias and results in an increasedpressure differential across the control ports and a proportional risein pressure at output 12. This feedback signal at output 12 of anamplifier 22 representing the effective friction brake force isconnected in parallel to control inputs 25 of Schmitt Triggers 18 and 19in the respective application and release networks.

When the feedback signal at input 25 of Schmitt Trigger l8 risesslightly above the effective friction brake control signal at input 24,indicating that the friction brake call has been satisfied, the SchmittTrigger 18 will turn off; i.e., the signal at output 26 will dissipatedue to the Schmitt Trigger switching to its opposite digital state. ORgate 23 is consequently disabled by the resultant absence ofa signal atcontrol port 28, causing the signal at control port 34 of valve 20 todissipate. Loss of pressure at port 34 results in fluid pressurecommunication being interrupted between input 35 and output 36 toterminate the brake application at a value corresponding to the frictionbrake control signal which is the difference between the load weighedbrake control signal and that portion of the brake request fulfilled bythe dynamic brake.

In order to prevent the feedback signal from exceeding the brake controlsignal at input 24 of Schmitt Trigger 19 in the release network, so asnot to inadvertently change its digital state, a choke 38 is placed inthe line connecting output 12 of amplifier 22 to input 25 of releaseSchmitt Trigger 19. In series with and downstream of choke 38 is avariable bleed choke 39 for adjusting the feedback signal at port 25 soas to establish the signal level a predetermined amount below thefriction brake control signal level at port 24. Within a rangeproportional to this pressure differential, the brake applicationpressure is able to overshoot the desired pressure without the feedbacksignal at port 25 exceeding the control signal at port 24. This permitsthe application network to be positively disabled without inadvertentlytriggering the Schmitt Trigger 19 and assures that the system will notcycle.

Since the state of the release Schmitt Trigger 19 remains stable, itwill be apparent that valve 21 also remains in its normal condition inwhich the brake application pressure is cut off from atmosphere.Consequently, a lap condition of the friction brakes is achieved whereinthe level of brake application pressure is maintained at a valuecorresponding to the friction brake control signal. Any subsequentimbalance or change in this steady state condition due, for example, toa rise or drop in the brake control signal or the brake applicationpressure will result in the control circuit responding, as the changingconditions dictate, to achieve and maintain the desired brakeapplication pressure, i.e., to supply or exhaust brake pressure. It willbe apparent therefore that this lap condition is the normal condition ofthe circuit.

With the application and release networks of the brake control circuitsin a lap condition, any change in the friction brake control signal willresult in the control circuit responding to vary the brake pressureaccordingly. For example, a further increase in the brake control signalwill again activate the application network through operation of SchmittTrigger 18, causing a further increase in brake application pressure aspreviously explained without changing the state of the release network.When the feedback signal level corresponding to the buildup of brakepressure is sufficient to overcome the control signal across SchmittTrigger 18, the application network will again become disabled to effecta lap condition of the brakes.

On the other hand, a reduction in the friction brake control signalwhich is effective at control inputs 24 of Schmitt Triggers 18 and 19associated with the respective application and release networks resultsin the release network being enabled through Schmitt Trigger 19 toexhaust brake pressure. Due to the signal at input 24 of Schmitt Trigger19 being reduced below the feedback signal at input 25, Schmitt Trigger19 is consequently switched to its opposite digital state in whichoutput 26 is depressurized and output 27 is pressurized. From output 27,control port 34 of relay valve 21 is pressurized to pilot the relay andconsequently connect brake pressure to atmosphere via its input port 35and output port 36 until the brake pressure corresponds to the brakecontrol signal. Input of amplifier 22 senses the brake pressureeffective at brake units 6 so that as the brake pressure reductionoccurs, the amplifier output 12 develops a proportional analog signal.Being effective at control inputs 25 of the Schmitt Triggers 18 and 19,the reduction of brake pressure will continue only until the feedbacksignal at input 25 of Schmitt Trigger 19 drops below the brake controlsignal at input 24, causing the Schmitt Trigger 19 to switch states.Relay valve 21 is accordingly disabled to terminate the flow of brakepressure to atmosphere.

During this cycle of events, the application network remains disableddue to the reduction in the brake control signal at input 24 of SchmittTrigger 18 simply reinforcing the disabled condition of the SchmittTrigger. Due to the series chokes 38 and 39, reducing the feedbacksignal at port 25, it will be apparent that the Schmitt Trigger 19 isable to anticipate the control signal at port 24 being satisfied. Thereduced feedback signal as the reduction in brake pressure approachesthe pressure called for by the brake control signal consequently causesSchmitt Trigger 19 to terminate the reduction in brake pressure beforethe reducing feedback signal is able to allow Schmitt Trigger 18 toSwitch on." Any hunting due to overshooting of the brake pressure, whichwould otherwise cause the system to cycle, is thus avoided by theanticipatory action of Schmitt Trigger 19.

During normal operation, as above described, control signals areprovided at control ports 30 and 31 to maintain NAND output 32 ofAND/NAND gate 8 depressurized, consequently withholding a pressure statecontrol signal from input 33 of OR gate 23 which is thereby conditionedto control supply valve 20 in response to control signals at input 28provided by Schmitt Trigger 18 which, together with release SchmittTrigger 19, is subject to feedback control by amplifier 22 forregulating the system brake pressure in a closed loop mode. In brakeapplication as well as lap conditions, outputs 26 of release SchmittTriggers 19 in brake control circuits 4 and 5 are pressurized toestablish control signal pressure at control ports 30 and 31 of AND/NANDgate 8, thereby depressurizing output 32. In brake release condition,outputs 26 of release Schmitt Triggers 19 are depressurized. However, avolume reservoir 40 in line 41 connecting the release Schmitt Triggeroutput 26, associated with circuit 4, to input 30 of AND/NAND gate 8provides a head of pressure to maintain control port 30 pressurized fora predetermined duration. A choke 42 prevents the driving pressureprovided by reservoir 40 from venting back through Schmitt Trigger 19.Similarly, a reservoir 40 and choke 41 provided in line 43 connectingthe release Schmitt Trigger output 26, associated with circuit 5, withAND/NAND gate control port 31 maintain control port 31 pressurized. Itwill thus be apparent that the size of volume reservoirs 40 is selectedto provide driving pressure control signals at AND/NAND gate 8 forsufficient duration until the normal circuit operation returns releaseSchmitt Triggers 19 to their normal state as established in lapcondition of the circuit, in which state outputs 26 charge reservoirs 40to maintain AND/NAND gate 8 disabled, thus preventing false failsafeoperation as long as the Schmitt Triggers return to their normal statewithin a predetermined period to give evidence of their integrity ofoperation.

In the event an unsafe operating condition arises due, for example, toone or the other, or both of the Schmitt Triggers 19 associated withbrake control circuits 4 and 5 being stuck in a release condition, inwhich outputs 26 are depressurized for such duration that the delayperiod provided by reservoirs 40 expires, as evidenced by loss ofpressure signals at either one or both of AND/NAND gate inputs 30 and31, NAND output 32 thereof will become pressurized. Consequently, acontrol signal is presented at input 33 of each OR gate 23, resulting intheir outputs 29 being pressurized to actuate supply valves 20, whichconnect supply pressure to brake units 6 at each truck. OR gates 23,being enabled by pressure supplied to control port 33, will maintainvalves 20 in an actuated state, allowing maximum supply pressure todevelop irrespective of continued control through the applicationSchmitt Triggers 18.

The fact that one or both release Schmitt Triggers malfunction bysticking in a release state in which outputs 26 are pressurizedindicates that the corresponding release or exhaust valve 21 isactuated, connecting brake pressure to atmosphere. A choke 44 at port 36of valve 21 limits this rate of exhaust so that full supply pressure,which is traditionally maintained at a value 50 percent higher than theexpected maximum brake pressure, should be sufficient to develop brakepressure to a value between 50 percent and 75 percent of normal maximumbrake pressure at the truck circuit having the malfunction. Since theredundant brake circuits would not likely malfunction at the same time,brake pressure at the truck opposite the one having a malfunction wouldbe expected to develop full brake pressure in response to themalfunction signal so that the total brake effort provided under suchunsafe operating conditions would be considered adequate to bring thevehicle to a safe stop.

I claim:

1. A vehicle brake control system including:

a. means providing a first signal representing a desired degree of brakeeffort,

b. means responsive to vehicle lead for modifying said first signal toproduce a primary brake signal,

c. means for generating a second signal representative of the degree ofdynamic brake effective in response to said primary brake controlsignal,

d. comparison means providing a secondary brake control signal inaccordance with the difference between said primary brake control signaland said second signal, and

e. brake application and release control means responsive to saidsecondary brake control signal, comprising:

i. supply valve means for controlling fluid pressure communicationbetween friction brake and a source of fluid pressure,

ii. exhaust valve means for controlling fluid pressure communicationbetween friction brake and atmosphere,

iii. means responsive to the degree of application of a friction brakefor providing a feedback signal proportionate thereto,

iv. first pure fluid digital amplifier means having opposing controlports subject to said secondary brake control signal and said feedbacksignal for producing an output signal when a difference therebetweenexists in one sense to pilot said supply valve means and thereby tend toreduce the difference between said secondary brake control signal andsaid feedback signals, and

v. second pure fluid digital amplifier means having a first and a secondoutput and opposing control ports subject to said secondary brakecontrol signal and said feedback signal, the difference therebetweenproducing a signal at said first output when in said one sense and atsaid second output when in a sense opposite said one sense to pilot saidexhaust valve and thereby tend to reduce the difference between saidsecondary brake control and said feedback signals.

2. A vehicle brake control system as recited in claim 1 wherein saidbrake application and release control means further comprises pure fluidOR gate means having one input subject to the output signal of saidfirst pure fluid digital amplifier for producing an output signal whenthe difference between said secondary brake control signal and saidfeedback signal is in said one sense to pilot said supply valve means.

3. A vehicle brake control system as recited in claim 2 wherein saidvehicle includes a plurality of trucks each having associated therewithsaid brake application and release control means, said system beingfurther characterized by failsafe control means comprising pure fluidAND gate means having a pair of inputs, each input being connected tothe first output maintaining} signal thereat for a predeterminedduration when said irst output of the respective one of said second purefluid digital amplifiers associated with said duplicate application andrelease control means is depressurized subsequent to pressurization ofsaid second output.

5. A brake control system as recited in claim 1 further comprising fluidpressure flow restriction means for modifying said feedback signal atsaid second pure fluid digital amplifier relative to said feedbacksignal provided at said first pure fluid digital amplifier to establisha differential set point at which said first and second pure fluiddigital amplifiers change state.

6. A vehicle brake control system comprising:

a. pure fluid proportional amplifier means for providing a first signalrepresenting a desired degree of brake effort,

b. fluid flow restriction means varied in accordance with vehicle loadfor providing a primary brake control signal,

c. means for generating a second signal representative of the degree ofdynamic brake effective,

d. pure fluid proportional amplifier means for providing a secondarybrake control signal in accordance with the difference between saidprimary brake control signal and said second signal, and

pure fluid brake control means for control of friction brakes inaccordance with said secondary brake control signal.

. A vehicle brake control system comprising:

a. means for providing a first signal representing a desired degree ofbrake effort on said vehicle,

b. means for generating a second signal representing the degree ofdynamic brake effective in response to said first signal,

c. comparison means for providing a friction brake signal in accordancewith the difference between said first and second signals, and

d. friction brake control means comprising:

i. supply valve means for controlling fluid pressure communicationbetween a friction brake and a source of fluid pressure,

ii. exhaust valve means for controlling fluid pressure communicationbetween said friction brake and atmosphere,

iii. a pure fluid proportional amplifier having an output pressurized inaccordance with the degree of pressurization of said friction brake toprovide a feedback signal, and

iv. application and release control means responsive to the differencebetween said friction brake signal and said feedback signal forcontrolling said supply and exhaust valve means to thereby control thedegree of friction brake application.

8. A vehicle brake control system, as recited in claim 7, wherein saidfriction brake control means further comprises failsafe control meansfor controlling fluid pressure communication between said source andsaid friction brake in bypass of aid application and release controlmeans when a malfunction of said friction brake control means occurs.

1. A Vehicle brake control system including: a. means providing a firstsignal representing a desired degree of brake effort, b. meansresponsive to vehicle lead for modifying said first signal to produce aprimary brake signal, c. means for generating a second signalrepresentative of the degree of dynamic brake effective in response tosaid primary brake control signal, d. comparison means providing asecondary brake control signal in accordance with the difference betweensaid primary brake control signal and said second signal, and e. brakeapplication and release control means responsive to said secondary brakecontrol signal, comprising: i. supply valve means for controlling fluidpressure communication between friction brake and a source of fluidpressure, ii. exhaust valve means for controlling fluid pressurecommunication between friction brake and atmosphere, iii. meansresponsive to the degree of application of a friction brake forproviding a feedback signal proportionate thereto, iv. first pure fluiddigital amplifier means having opposing control ports subject to saidsecondary brake control signal and said feedback signal for producing anoutput signal when a difference therebetween exists in one sense topilot said supply valve means and thereby tend to reduce the differencebetween said secondary brake control signal and said feedback signals,and v. second pure fluid digital amplifier means having a first and asecond output and opposing control ports subject to said secondary brakecontrol signal and said feedback signal, the difference therebetweenproducing a signal at said first output when in said one sense and atsaid second output when in a sense opposite said one sense to pilot saidexhaust valve and thereby tend to reduce the difference between saidsecondary brake control and said feedback signals.
 2. A vehicle brakecontrol system as recited in claim 1 wherein said brake application andrelease control means further comprises pure fluid OR gate means havingone input subject to the output signal of said first pure fluid digitalamplifier for producing an output signal when the difference betweensaid secondary brake control signal and said feedback signal is in saidone sense to pilot said supply valve means.
 3. A vehicle brake controlsystem as recited in claim 2 wherein said vehicle includes a pluralityof trucks each having associated therewith said brake application andrelease control means, said system being further characterized byfailsafe control means comprising pure fluid AND gate means having apair of inputs, each input being connected to the first output of saidsecond pure fluid digital amplifier means associated with a differentone of said trucks and a NAND output producing a signal for connectionwith a second input of said OR gate when one or both inputs of said ANDgate are absent.
 4. A vehicle brake control system as recited in claim 3wherein said failsafe control means further comprises signal delay meansassociated with each input of said AND gate for maintaining a signalthereat for a predetermined duration when said first output of therespective one of said second pure fluid digital amplifiers associatedwith said duplicate application and release control means isdepressurized subsequent to pressurization of said second output.
 5. Abrake control system as recited in claim 1 further comprising fluidpressure flow restriction means for modifying said feedback signal atsaid second pure fluid digital amplifier relative to said feedbacksignal provided at said first pure fluid digital amplifier to establisha differential set point at which said first and second pure fluiddigital amplifiers change state.
 6. A vehicle brake control systemcomprising: a. pure fluid proportional amplifier means for providing afirst signal representing a desired degree of brake effort, b. fluidflow restriction means varied in accordance with vehicle load forprovidiNg a primary brake control signal, c. means for generating asecond signal representative of the degree of dynamic brake effective,d. pure fluid proportional amplifier means for providing a secondarybrake control signal in accordance with the difference between saidprimary brake control signal and said second signal, and e. pure fluidbrake control means for control of friction brakes in accordance withsaid secondary brake control signal.
 7. A vehicle brake control systemcomprising: a. means for providing a first signal representing a desireddegree of brake effort on said vehicle, b. means for generating a secondsignal representing the degree of dynamic brake effective in response tosaid first signal, c. comparison means for providing a friction brakesignal in accordance with the difference between said first and secondsignals, and d. friction brake control means comprising: i. supply valvemeans for controlling fluid pressure communication between a frictionbrake and a source of fluid pressure, ii. exhaust valve means forcontrolling fluid pressure communication between said friction brake andatmosphere, iii. a pure fluid proportional amplifier having an outputpressurized in accordance with the degree of pressurization of saidfriction brake to provide a feedback signal, and iv. application andrelease control means responsive to the difference between said frictionbrake signal and said feedback signal for controlling said supply andexhaust valve means to thereby control the degree of friction brakeapplication.
 8. A vehicle brake control system, as recited in claim 7,wherein said friction brake control means further comprises failsafecontrol means for controlling fluid pressure communication between saidsource and said friction brake in bypass of aid application and releasecontrol means when a malfunction of said friction brake control meansoccurs.